In some electro-statographic printing techniques, the printing process begins with placing a uniform electrostatic charge on a photoreceptor and exposing the photoreceptor to a light and shadow image or to a scanning laser to dissipate the charge on the areas of the photoreceptor exposed to the light and developing to form a latent electrostatic image. The resultant latent image is developed by subjecting the latent image to a liquid toner comprising a carrier liquid and pigmented toner particles. These toner particles are generally comprised of a pigmented polymer. Generally, the development is carried out, at least partially, in the presence of an electric field, such that the toner particles are attracted either to the charged or discharged areas, depending on the charge of the particles and the direction and magnitude of the field.
This image may then be transferred to a substrate such as paper or plastic film, often via an intermediate transfer member (“ITM”) which is typically covered with a replaceable print blanket. The transferred image may then be permanently affixed to the substrate by the application of pressure, heat, solvent, overcoating treatment or other affixing processes. In general, in the commercial process used by HP-Indigo, the ITM is heated to a temperature that causes the toner particles and residual carrier liquid to form a film in the printed areas which is transferred to the final substrate by heat and pressure. Fixing to the final substrate is part of the transfer process.
The use of ITMs, and ITMs including print blankets, is well known. One disadvantage of using print blankets in electro-static printing is called “gloss memory”. Gloss memory is observed when the same image is repeatedly printed on the same area of a print blanket. After a certain number of print cycles, the number depending on variables such as the type of print blanket and toner, the gloss on the print blanket where the image was printed is different than on the areas where it wasn't. Gloss memory manifests itself in subsequent printings of different images by producing images that vary in gloss depending on the image which caused the gloss memory. Repetitive printing of the same image can also affect the optical density memory of the print blanket and/or photoreceptor and the effectiveness of transfer of small dots in images.
Various attempts have been made to solve the gloss memory failure of print blankets in electro-static printing. The attempts have included advances in techniques of printing as well as in the equipment and materials used. For example, a technique has been developed whereby a solid color page, sometimes referred to as a “sky shot” in the art, is printed after a to predetermined number of printings. The idea is that the comprehensive layer of toner that is deposited on the blanket acts as a cleanser, adhering to stray toner particles and other debris and carrying them along for affixation to a final substrate material, such as paper. A disadvantage of the technique, however, is that the sky shot wastes toner and substrate material.
Another attempted solution to the gloss memory problem derives from the blanket itself. Conceivably, a blanket could be developed which resists gloss memory altogether. However, in practice it has been found that a print blanket that is resistant enough to significantly reduce gloss memory becomes ill-suited for liquid electro-static printing. Another equipment innovation that has been developed for addressing the gloss memory problem involves the liquid toner formulations that are used. ElectroInk® 4.0, which was developed by HP-Indigo® and which is commercially available, is such a liquid toner. However, gloss memory, even when printing is carried out with the improved ElectroInk® toner formulations, is still a problem for the field.